To be honest, I don’t know what technology is used. I know that sweepers look in the obvious places - in lamps, ceiling fixtures, on bookshelves, in plants, etc. But those are physical searches. There may be (no doubt are) technology devices that search for electronic pulses, etc., that may be emitted from hidden devices.
Video recorders are much easier to find than audio recorders because they (video recorders) must obviously have line of sight, so they tend to be more visible. But audio recorders don’t require line of sight and they can get tucked away in all kinds of odd places.
I was watching something on Netflix the other night - a Longmire episode, I think - and the sheriff found a video recorder in the mouth of a deerhead mounted behind the counter in a gunshop. So that is one instance where a video camera was in a bit of an odd place. But I remember seeing on another program not too long ago where an audio recorder was found mounted beneath the edge of a bathroom cabinet - where it had been for years without ever being detected.
Edited to add - let’s don’t forget that RG was recording conversations from his phone in his pants pocket.